Long‐term measurements (1974–1993 and 1996, respectively) of the net radiation (Q), global radiation (G), reflected global radiation (R), long‐wave atmospheric radiation (A) and thermal radiation (E) of a pine forest in Southern Germany (index p) and of a grass surface in Northern Germany (index g) are compared. The influence of changes in surface properties is discussed. There are, in the case of the pine stand, forest growth and forest management and in the case of the grass surface, the shifting of the site from a climatic garden to a horizontal roof. Both series of radiant fluxes are analyzed with respect to the influences of the weather (cloudiness, heat advection). To eliminate the different influence of the solar radiation of the two sites, it is necessary to normalize by means of the global radiation G, yielding the radiation efficiency Q/G, the albedo R/G=α and the normalized long‐wave net radiation (A+E)/G. Furthermore, the long‐term mean values and the long‐term trend of yearly mean values are discussed and, moreover, a comparison is made of individual monthly values. Qp is twice as large as Qg. The reason for this is the higher values of G and A above the pine forest and half values of αp compared to αg. Ep is only a little greater than Eg. The time series of the radiation fluxes show the following trends: Qp declines continuously despite a slight increase of Gp. This is mainly due to the long‐wave radiation fluxes. The net radiation of the grass surface Qg shows noticeably lower values after the merging of the site. This phenomenon is also dominated by the long‐wave radiation processes. Although the properties of both site surfaces alter, Ep and Eg remain relatively stable. Ap and Ag show a remarkable decrease however. The reason for this is to be found in a modification of the heat advection, showing a more pronounced impact on the more continentally exposed site (pine forest). Compared to αg, αp shows only a small variability. The changes of αp can be easily explained as resulting from the influence on the albedo of alterations in cloudiness and the changes in the water storage in stand and soil produced by varying weather conditions. The influence of the forest management is only small and short‐termed. αg reacted with a leap on site merging, followed by a continuous drop. The drop of the grass surface albedo αg on top of the observatory is probably related to the gradual development of the newly sown lawn there and the resultant change in the specific hydrological balance of the soil. A decreasing radiation efficiency Q/G at both sites is observed, which is principally due to an increase in −(A+E)/G. The investigation discovered changes in the long‐wave radiation processes. Its influence in Q should lead to more attention to the long‐wave radiation processes in routine monitoring of radiation. Copyright © 1999 Royal Meteorological Society
SummaryThe experimental site of the Department of Meteorology of Freiburg University at the Hartheim pine stand is first described. There, since 1973 long term measurements of net radiation and its components have been carried out. In addition we have been monitoring the different heat fluxes and components of the forest water budget.From May 1 1 th to May 24th 1992 a special international and interdisciplinary observation period was organized in Hartheim (HartX 92). This took place in the frame of the international regional climatic project REKLIP (Regionales Klima Projekt). We then describe the permanent equipment and the special HartX installations. After that we show the climate of the region, in May 1992 and the weather during the HartX period. It was extraordinarily warm and the precipitation was much less than normal. The cloud cover was very small.We report the results of the radiation measurements (net radiation and its components). They are compared to the long standing measurements (1974)(1975)(1976)(1977)(1978)(1979)(1980)(1981)(1982)(1983)(1984)(1985)(1986)(1987)(1988). Moreover the longstanding data of the components of the water budget (throughfall, canopy drip and stemflow, interception and transpiration) of the period 1978-1985 are dealt with. In addition we report the behaviour of the energy fluxes (soil-stand heat flux, turbulent sensible and latent heat fluxes) of the period 1974-1988. These estimations are compared to the conditions in May 1992 and the conditions during HartX 92.
des Altiplano wahrend der letzten Vereisung 165 Yoshino, M. M.: 1959; A micro-climatological study of surface winds affected by micro-topography. ?Proc. IGU Reg. Conf. Japan 1957." 243?249. Yoshino, M. M.: 1960; The local distribution of wind shaped trees and Usnea as indicators of microclimates in the subalpine zones.
The Hartheim pine forest is an ecosystem in the dry region of the southern upper Rhine valley in Germany. A project "Monitoring the Hartheim pine forest" was initiated in 1973 and is continuing. One of its objectives is to provide long-term measurements of the net radiation and its components, to calculate the energy fluxes and other hydrological parameters.Sufficient studies on the climatology of the 'primitive' elements of climate exist in central Europe. But long-term series of measurements of the physical processes controlling these elements above different characteristic surface types are missing. The study of heat balance monographies (e.g. Budyko, 1974;Miller, 1981;Kessler, 1985;Hantel, 1989;Baumgartner, 1990) illustrates this deficit drastically. An aspect of our investigations is to study the long-term behaviour of the above-mentioned complex physical variables in relation to weather and climate. Furthermore, we examine whether it is possible to distinguish between the changes, for example, of the ratio of net radiation to global radiation caused by the dynamics of climate and the changes caused by alterations of the ecosystem. Natural and human influences on the pine stand at Hartheim led to an 8-m growth in height during the monitoring period.We present some examples showing that the dynamics of the weather hide other effects within the frame of 20 years of measurements. On the other hand, evident changes in interception and transpiration are due to a changing forest ecosystem.
SummaryDuring two measurement campaigns in 1992 (the Hartheim Experiment HartX-and an additional experiment in autumn). measurements of soil moisture were carried out in a Pinus sylvestris stand at Hartheim on the Oberrhein. Several methods were used to determine soil water status. They were compared in terms of suitability for estimating stand evapotranspiration (ET) via soil water depletion. Measurements of tree water potential suggested that conductance of the trees was affected by soil water depletion during the period of the HartX campaign in spring 1992. We interpret the observations to indicate a lesser influence of soil water availability on tree transpiration during the autumn experiment.Eddy correlation and xylem sapflow measurements provided reference ET values with which to compare the stand ET calculations based on soil moisture measurements. Profile measurements of soil moisture showed that particularly in springtime when the lower soil layers are saturated with water, the water transport from depths below the major rooting zone is a very important factor affecting evaluation of stand ET. Decreases in soil water storage may be determined best with permanently installed soil moisture sensors such as used in tensiometric or TDR measurements that provide high resolution of changes over time.
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